This invention relates generally to inspecting an electrical connection, or splice, between a power lead and a magnet wire and, more particularly, to methods and apparatus which may be utilized in determining the acceptability of such a splice.
Dynamoelectric machines, such as motors and generators, typically include a magnetic core having a plurality of windings formed from magnet wire. For example, in one particular configuration, an electric motor includes a stator core having a stator bore. Side portions of a plurality of stator windings formed by the magnet wire are inserted into stator core slots having open ends at the periphery of the stator bore, and each end of the magnet wire extends from the stator core and is electrically connected, or spliced, to at least one power lead.
Magnet wire of a stator typically has a tough enamel coating. Such coating may enhance motor performance, for example, by electrically insulating each winding turn from other winding turns and protecting the magnet wire against damage which may cause reduced operational efficiency due to increased resistance of the magnet wire and possibly even short circuiting of the wire.
Although the enamel coating may enhance motor operation, such coating generally presents difficulties when attempting to form a reliable electrical connection, or splice, between a magnet wire and a power lead. For example, even though the insulation at the ends of the power leads may be stripped away so that the power lead conducting wires are exposed, it is time consuming and difficult to remove a segment of the enamel coating from each end of the magnet wires.
To reduce the time required to make electrical connections between stator magnet wires and power leads, a connector formed of electrically conducting material may be utilized. One known connector is, for example, substantially U-shaped and has a plurality of sharp serrations formed in the connector material on the interior surface of the connector. The stripped end of at least one power lead and one end of an enamel coated magnet wire may be placed within the interior of the U-shaped connector. A crimper may then fold the legs of the connector over the ends of the magnet wire and power lead, and may also squeeze, or crimp, the connector so that the sharp serrations are forced through the magnet wire enamel coating and into electrical contact with the magnet wire. Such crimping of the connector also may ensure that the connector is in electrical contact with the conducting wire at the stripped end of the power lead. An insulating sheath, which may be formed of heat-shrinkable insulating material, may then be placed over the crimped connector and heated so that the sheath shrinks and grips the connector.
Although the above described connector and crimping process generally form an acceptable electrical connection, or splice, between the magnet wire and power lead, there is a possibility that the magnet wire may xe2x80x9cfloat-upxe2x80x9d, or move within the connector toward the open end of the connector legs, during the crimping process. As a result, the serrations may not make good electrical contact with the magnet wire. For example, the serrations may not extend fully through the enamel coating and into firm contact with the magnet wire. If good electrical contact is not made between the magnet wire and the connector, motor performance may be adversely affected, for example, due to increased resistance and power loss at the connector.
Further, since manufacturing limitations may prevent high magnitude current stator winding testing, a bad electrical connection between a magnet wire and a power lead may not necessarily be detected until the motor actually is put in the field. High current testing generally is not performed on stator windings since such testing itself could damage the magnet wire. Also, even though a stator may pass low current tests at the manufacturing site, the vibrations and normal external conditions which a motor is exposed to during shipping may further deteriorate a bad electrical connection. As a result, the motor may not pass even low current tests when performed at the delivery site.
Early detection of bad electrical connections, especially prior to delivery, may facilitate reducing costs by avoiding costs associated with having motors deemed unacceptable at the delivery site or in the field due to bad electrical connections. Such early detection may also facilitate enhancing customer confidence.
With respect to the detection of unacceptable electrical connections, since such detection may be performed at a manufacturing site, it would be preferable to provide a manner of detecting such unacceptable connections which does not require substantial training and can be easily performed. In addition, rather than a mere qualitative, e.g., bad or good, measurement, a quantitative measurement indicative of the nature of an electrical connection may be preferred. A quantitative measurement may be more suitable, for example, for statistical process control applications in a manufacturing setting. For example, depending on the quantitative temperature measurement, a harder crimp may be required to form an acceptable splice. Further, to avoid an unacceptable increase in manufacturing time, such detection preferably would be performed rapidly. Such rapid detection would be particularly crucial in high volume manufacturing operations.
Accordingly, it would be desirable to improve motor reliability by facilitating the identification, based on quantitative measurements, of unacceptable electrical connections between stator magnet wires and power leads. It would also be desirable and advantageous to identify such unacceptable electrical connections without significantly increasing the costs and time associated with manufacturing a stator.
An object of the present invention is to improve motor reliability by facilitating early identification, based on quantitative measurements, of potentially unacceptable electrical connections between stator magnet wires and power leads.
Another object of the present invention is to quickly, and at a low cost, identify such potentially unacceptable electrical connections.
Still another object of the present invention is to facilitate reducing costs by reducing the quantity of motors which may be returned due to unacceptable electrical connections between stator magnet wires and power leads.
Yet another object of the present invention is to facilitate enhancing customer confidence by better ensuring that reliable electrical connections are made between stator magnet wires and power leads in delivered motors.
These and other objects may be attained by apparatus and methods for inspecting electrical connections, or splices, between stator magnet wires and power leads which, in one embodiment of the apparatus, includes a processing unit, a power supply unit, and a temperature sensing unit. The processing unit, in one embodiment, includes a programmable logic controller (PLC) having a central processing unit (CPU) and a plurality of input and output slots. The power supply unit, in one embodiment, includes a power lead connector designed to interconnect the motor power leads and a power control relay. The relay is coupled to and controlled by the PLC. The temperature sensing unit, in one embodiment, includes infrared thermometers and probes for sensing the temperature at the electrical connections, or splices, between the stator magnet wires forming the motor windings and the power leads. The outputs of the thermometers are coupled to an input slot of the PLC.
In one form of operation, and to determine whether a particular electrical connection is unacceptable, i.e., to determine the presence of electrical connection faults, the temperature sensing probe is positioned sufficiently near the electrical connection to sense or measure temperature at the connection. The operator may then depress a manual start switch, and the PLC causes the power relay to close and the motor windings are energized. While the windings are energized, the temperature sensing unit generates electrical signals representative of the temperature at the electrical connection, and such signals are supplied to the PLC. The CPU compares the signals received from the temperature sensing unit with at least one predetermined value stored in a suitable memory element. The predetermined value represents an upper temperature limit for an acceptable connection.
If the sensing unit output signal is above the predetermined value, then the connection may be unacceptable and the PLC generates a fault signal to energize a fault indicator, e.g., a light emitting diode (LED), to alert the operator that an electrical connection fault, such as a bad splice, may have been identified. Except as noted below, if the sensor output signal is below the predetermined value, then the connection is determined to be acceptable and no fault signal is generated. If the sensor output signal is substantially unchange from just prior to energization of the motor windings to the time at which the windings are energized, and even if the signal is below the predetermined value, then the connection may be unacceptable, e.g., an open circuit, and the PLC generates a fault signal.
The apparatus and methods described above improve motor reliability by facilitating the identification, based on quantitative measurements, of unacceptable electrical connections between stator magnet wires and power leads. Such apparatus and methods also enable identification of such unacceptable electrical connections without significantly increasing the costs associated with manufacturing a stator.